High density composite focal plane array

ABSTRACT

A composite focal plane assembly with an expandable architecture has a multi-layer, double-sided aluminum nitride (AlN) substrate and vertical architecture to achieve the dual function of focal plane and electronics backplane. Imaging dice and other electrical components are mounted and wire bonded to one surface and then direct backplane connectivity is provided on the opposing surface through a matrix of electrical contacts. In one embodiment, a flexible connector is sandwiched between the AlN focal plane and a FR-4 backplane is used to compensate for differences in coefficient of thermal expansion (CTE) between the AlN and commercially available high density circuit card connectors that are commonly manufactured from materials with CTE properties more closely approximating FR-4. In an alternate embodiment, the FR-4 and flexible connectors are eliminated by using high density circuit card connectors that are fabricated out of materials more closely matching the CTE of AlN.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a divisional of U.S. application Ser. No.12/327,383, filed Dec. 3, 2008, and claims rights under 35, USC §119(e)from U.S. Application Ser. No. 61/193,258, filed Nov. 12, 2008, thecontents of which are incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH

The present invention was made with United States Government supportunder Contract No. FA8650-07-C-7732. The United States Government hascertain rights in the present invention.

1. Technical Field

The present invention relates to a very large format imaging focal planearray with an expandable architecture that can be used to make agigapixel camera. More particularly, the invention relates to anapparatus that consolidates the functions of a large scale Focal Plane,signal distribution substrate, heat sink and motherboard into a singleplanar assembly having a vertical architecture wherein the signal leadsfrom hundreds of active, video and passive components mounted on the topsurface of the focal plane are routed through a multi layer, thermallyconductive, flat and stiff circuit card substrate to the hack surfacewhere the signal lines are formed into a matrix of electrical contactsthat can be connected directly or nearly directly to a group ofconnectors serving as a backplane interface to a set of processingelectronics circuit cards.

2. Description of Related Art

Imaging dice are commonly mounted singularly into a device carrierpackage or sometimes in small groups onto a common substrate. Electricaldie connections are then wire bonded to contacts either within the chipcarrier or down to the substrate where they are typically routed to thesides of the focal plane assembly away from the active centers of thedie to sites along periphery where additional, connections can be madeto external circuitry. The external circuits are commonly connectedusing either flex or wired cables.

However, when the size of the imaging plane board is limited, and alarge number of imaging die are required, the prior art construction ispushed beyond its limits in terms of size and the number of connectionswhich is achievable by making connections at the perimeter of theimaging plane board.

It is therefore desirable to provide an optically flat imaging planeboard having a small perimeter in proportion to the number of imagingdies placed upon the board.

It is likewise desirable that a large scale focal plane handling tens orhundreds of power consuming dice be thermally conductive in order todissipate the significant amounts of heat being generated by the devicesunder power.

SUMMARY

In accordance with one preferred embodiment of the invention, analuminum nitride (AlN) board is used in a composite focal plane array.The aluminum nitride board can be fabricated into a multi-layer, twosided circuit card assembly, can be post polished to optically flattolerances, is structurally stiff and is highly thermally conductivewhich enables it to dissipate large amounts of heat. Also, thecoefficient of thermal expansion of the AlN closely matches that ofimaging dice which are constructed out of silicon wafers. However, thealuminum nitride board has substantially lower coefficient of thermalexpansion (CTE) than conventional FR-4, and similar fiber glassmaterials used most commonly for commercial circuit card construction.Additionally, manufacturers of high density backplane connectorstypically fabricate their products out of Liquid Crystal Polymer (LCP)or similar materials to more closely match the CTE of the FR-4, andother like materials. In this embodiment of the invention, a flexibleconnector array is connected between the imaging plane board backsurface and the motherboard. The flexible connector allows compensationfor the difference in the coefficient of thermal expansion. The imagingdies are connected to vias (electrically conducting paths) in theimaging plane board by Tire bond connections. The configuration of thedie wire bond connections to the vias in the imaging plane board arearound the perimeter of each imaging die. The imaging plane boardprovides for distribution of signals and via paths through multiplelayers of the board, which provide for an array of connections on thebackside of the board. A land grid array (LGA) type connector or otherflexible connector is then used to connect the imaging plane board to aback plane motherboard. Use of an LGA type connector provides a flexibleconnector between the FR-4, motherboard and the AlN imaging plane board,which compensates for differences in thermal coefficient expansion.

All features and advantages of the present invention will becomeapparent in the following written detailed description and claims.

In a second embodiment of the invention 8, an aluminum nitride (AlN)board is used in a composite focal plane array directly without anyintermediary FR-4 back plane.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and a preferred apparatus is shown not to scale. However,the advantages of the invention and further objects thereof will best beunderstood by the detailed description as well as the drawings.

FIG. 1 shows an assembly of the focal plane, the back plane, and theconnector of the invention. The drawing is not to scale and illustratesthe construction of the preferred embodiment.

FIG. 2A shows a top view of the focal plane imaging plane board wherethe imaging dies are mounted. The imaging dies are shown.

FIG. 2B shows a view of the back surface of the imaging plane board withorderly rows of contact pads.

FIG. 3A shows a silicone carbide support structure which is used tosupport connectors between the imaging plane board and the back planemotherboard.

FIG. 3B shows the connections on the back plane motherboard.

FIG. 3C shows a stiffening plate which reinforces the FR-4, motherboardand which supports the back plane connectors.

FIG. 4 shows a compression type connector which connects the contactpads of the focal plane board and the contacts of the motherboard.

FIG. 5 shows internal layer traces of the focal plane board connection,which route vias between the top layer bond pads connecting the imagingdie to the imaging plane board and the array of contacts on the imagingplane board back surface.

FIG. 6A shows an assembled view of a composite focal plane arrayassembly.

FIG. 6B shows an exploded view of a composite focal plane arrayassembly.

FIG. 7 shows a complete focal plane assembly with the back planeassembly in a camera.

FIG. 8 shows a second embodiment of the invention which combines thefunctions of the focal plane board and the mother board.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a cross-sectional view of a high-density composite focalplane array with the integral back plane board and camera controlmodules. The optically flat focal plane board 10 is preferably made oflayers of aluminum nitride. A large number of imaging dies 14 are placedon the top 12 of the board 10. The number of dies may be enough tocomprise a gigapixel camera or a quadrant there-off as in thisembodiment. Connectors 16 are placed on the bottom side 18 of the focalplane board 10. The connectors 16 connect a back plane motherboard 20 tothe focal plane board 10. On top of the entire assembly is a window 22made of a material such as sapphire. A silicone plate 24 is placedbetween the imaging plane board and the motherboard and provides aspace, which is used to secure the connectors 16. The connectors 16 aresandwiched between the imaging plane board and the FR-4, motherboard 20.The camera assembly, which utilizes the focal plane array and integralback plane array, is also depicted in FIG. 1. SMT connectors 26 connectthe motherboard to camera control modules (processing electronics) 28.It should be noted that FIG. 1 is not to scale, and that configurationsof the preferred embodiment are shown in photographs, which arc FIGS.2-7. FIG. 1 shows the components of the assembly conceptually.

The connectors 16 provide for compensation for the difference betweenthe thermal expansion rate of the focal plane board 10 and themotherboard 20. The connectors are preferably spring-loaded connectorsof the type used for land grid array chips. Typical LGA connectorsprovide a spring-loaded contact against pads. Examples of suitableconnectors for this invention include those shown in the U.S. Pat. Nos.6,758,683; 6,585,527;, and 6,695,624, and Publication Numbers2006/0186906; 2003/0203664;, and 2004/019208. Further, a connector whichmay be used with this invention is depicted in FIG. 4. The connector 16is produced by SAMTEC and is identified as GLZ series. This is asolder-less interconnection type connector. As used in this invention,an example of the connector is an array of connections, which are 40connections by 10, connections for a total of 400, connections. SAMTECis located at 520 Park East Boulevard, New Albany, Ind. 47150. TheSAMTEC GFZ connectors are land grid array socket assemblies which mayalso be used for connecting boards, or as test devices for testingchips. This invention, however, is not limited to connectors as producedby SAMTEC. Any connector, which provides for contacts which will providefor compensation for the differences in thermal coefficient expansionbetween the imaging plane board and the back plane motherboard issuitable for use with this invention.

The optically flat imaging plane board 10 is preferably an aluminumnitride printed circuit board. The aluminum nitride board can provide anoptically flat imaging plane and when constructed with a plurality oflayers provides signal lines (vias) (electrical connections) through thethickness of the boards. As shown in FIG. 5, the vias 30 (white lines)overlap each other by passing through layers of the board 10 to allowtransition from imaging plane layer bond patterns of the imaging die toan array which matches the flexible connectors 16. The aluminum nitrideboard is currently available in a size of 4.5″×4.5″. With this sizeboard, it is not feasible to provide connections to a motherboard bywire bond connections at the edge of the board 10. The practicallimitation is that if the pattern of the imaging die on the surface issquared, than the number if imaging die increases as the square of thenumber on a side. On the other hand, the number of possible wire bondconnections around the perimeter of the imaging plane board onlyincreases linearly. For this reason, if a construction with a largenumber of dies is required, the number of wire bond connections to bemade at an edge of an imaging plane board quickly exceeds feasiblelimits on the size of the board.

FIG. 2A shows an array of 92, imaging die placed upon an imaging planeboard 10. In this figure, wire bonds 32 which connect the imaging dies14 to the imaging plane board 10 are shown. It can be easily seen thatthe large number of required connections may exceed the space isavailable for wire bond connections around the perimeter of board 10. Inthis invention, the vias (see FIG. 5) allow rearrangement of the patternof the wire bond contacts 32 to an array pattern 34 shown in FIG. 2B.FIG. 2B as noted above is the backside of the imaging plane board 10.The array shown in FIG. 2B is, in this example, designed to match anarray of a SAMTEC GFC type connector. However, it should be noted thatother connectors are suitable with this invention. There may beapproximately 5,000, wire bondable pads between the imaging die and theimaging plane board. The bottom of the board 10 may have approximately1500, pressure contact pads for engaging connector 16. However, theactual number can exceed 3000, allowing for a large number of imagingdies.

The connections 36 to the motherboard 20 are shown in FIG. 3B. Theconnections 36 engage the connector 16 to provide contact between themotherboard and the imaging plane board. FIG. 3A shows a frame, whichholds connectors 38, and a space 40, which receives connectors 16. Alsoshown in photograph FIG. 3A are the contacts 34 on the back of imagingplane board 10. FIG. 3C shows a stiffening board 42, which is used toreinforce the FR-4, motherboard 20. This stiffener also supports backplane connectors 36 as shown in FIG. 3B.

FIG. 6A shows a completed composite focal plane array assembly 50 andFIG. 6B shows an expanded view of the components of the assembly. Theimaging plane hoard 10, the imaging die 14 and the frame 38 previouslydiscussed are shown. In addition, there is shown a bezel 44, whichprovides separation between the imaging plane board and a sapphirewindow 48. A support ring 46 is also provided in the completed assembly.

FIG. 7 shows an overall layout of a camera assembly incorporating thecomposite focal plane assembly 50 depicted in FIG. 6, a plurality ofconnectors 16, a back plane motherboard 52 with contacts and cameraboards 54.

In the second embodiment (FIG. 8) the aluminum nitride board isfabricated into a multi-layer two sided circuit card assembly, which ispost polished to optically flat tolerances, is structurally stiff and ishighly thermally conductive which enables it to dissipate large amountsof heat. Also, the coefficient of thermal expansion of the AlN closelymatches that of imaging dice which are constructed out of siliconwafers. With the use of a backplane connector made from materials ofsimilar CTE to the AlN, including but not limited to AlN itself, adirect single layer apparatus is achieved wherein the processingelectronics are plugged directly into the AlN focal plane backplane,eliminating the need for the thermally compensating flexible connectorarray between the imaging plane board back surface and the motherboard.In this embodiment, the focal plane is the backplane with the imagingdie mounted on one surface and the processing electronics pluggeddirectly into connectors mounted on the other surface.

FIG. 8 shows the second embodiment of the invention where the aluminumnitride composite focal plane board 10 combines the functions of thefocal plane and the FR-4, mother board of the first embodiment (FIG. 1).In this ease, the coefficient of thermal expansion (CTE) of the aluminumnitride board 10 substantially matches the material of the back planeboard connectors 60 and 62. This eliminates the need for a thermallycompensating connector 16 which is used in the first embodiment (FIG.1). In the first embodiment, the FR-4, CTE is not closely matched to thealuminum nitride board.

While the application has been described in detail herein in accord withcertain preferred embodiments thereof, many modifications and changestherein may be effected by those skilled in the art. Accordingly, it isintended by the appended claims to cover all such modifications andchanges as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A focal plane and back plane assembly comprising:an aluminum nitride printed wiring board; a plurality of imaging diesmounted on the aluminum nitride printed wiring board; a plurality ofsignal lines routed through the aluminum nitride printed wiring board;at least one connector connected to the back of the printed wiring boardproviding an electrical connection to the signal lines of the aluminumnitride printed wiring board; and a back plane motherboard havingelectrical connections to the connector.
 2. The apparatus of claim 1wherein the plurality of imaging dies are mounted on an image receivingplane of the aluminum nitrite printed wiring board and the connector ismounted on the back opposite to the image receiving plane.
 3. Theapparatus of claim 1 wherein the motherboard is a FR-4 material.
 4. Ahigh-density focal plane assembly having an integral back planecomprising: an aluminum nitride printed -wiring board having a pluralityof imaging dies on a first surface and signal lines (vias) routedthrough the aluminum nitride printed wiring board to a second surface ofthe aluminum nitride printed wiring board; a back plane motherboardhaving electrical contacts; a connector sandwiched between the aluminumnitride printed wiring board and the motherboard wherein the connectorconnects the signal lines of the aluminum nitride printed wiring boardto the motherboard electrical contacts.